Substitution reactions of the complexes [(TLtBu)PtCl]+ and [Pt(tpdm)Cl]+, where TLtBu = 2,6-bis[(1,3-di-tert-butylimidazolin-2-imino)methyl]pyridine and tpdm = terpyridinedimethane, with nucleophiles: S-methyl-L-cysteine (S-Met-L-Cys), L-cysteine (L-Cys), glutathione (GSH) and L-methionine (L-Met) were studied in aqueous 0.1 M NaClO4 solution in the presence of 10 mM NaCl using variable-temperature UV-vis spectrophotometry. The higher reactivity of the complex with the tpdm ligand could be attributed to the influence of the bulkiness of the tert-butyl-groups from the [(TLtBu)PtCl]+ complex. The order of reactivity of the studied ligands is: S-Met-L-Cys > L-Met > GSH > L-Cys. The thioethers (S-Met-L-Cys and L-Met) are more reactive than the thiols (GSH and L-Cys). This order of reactivity is in relation with their electron properties and structures. The negative values reported for the entropy of activation confirmed the associative mode.